Great expectations have risen for application of a Ge-added Nb3Al-based superconducting wire as a wire which can be used in strong magnetic fields of 21 T or higher because it has higher upper critical magnetic field than practically used Nb3Sn superconducting wires.
As to production method of a Ge-added Nb3Al-based superconducting wire, it is general to directly diffuse Nb together with Al or Al alloy, and high critical magnetic field can be obtained by simply heating at elevated temperatures. However, such heat treatment at elevated temperatures makes resultant crystal particles bulky, and critical current density practically required cannot be realized. Furthermore, since a practical strong magnetic field magnet is requested to realize high transport current from the view point of protection from quenching or the like, a practical wire should essentially have not only high critical current density but also high transport current.
For improving characteristics of a Ge-added Nb3Al-based superconducting wire, two approaches have been proposed heretofore.
In the first approach, heat treatment is conducted at low temperature so as to prevent crystal particles from becoming bulky, while a diffusing pair of Nb and Al alloy, namely the size of alloy core is reduced as small as possible, for example, less than or equal to 1 μm to destabilize a phase which is an intermediate compound, in order to improve the stoichiometry of an Al5 phase which is a superconducting phase.
In the second approach, crystal particles are prevented from becoming bulky through short retention at an elevated temperature at which the Al5 phase of stoichiometric composition is stable, followed by quenching if necessary. Also in this approach, it is preferred to reduce the size of the diffusing pair of Nb and Al. This allows Nb and Al to react in a very short time.
As described above, in the conventional techniques, it is believed that the size of the diffusing pair of Nb and Al should be as small as possible.
However, workability of Nb—Al—Ge composite material is very poor, and it is very difficult to incorporate a diffusing pair of fine Nb and Al alloy in a precursor wire. An Al-dissolved material containing an amount of Ge required to sufficiently improve characteristics exhibits a typical eutectic structure, and is so difficult to be processed that a small work may cause a crack.
Therefore, it is the current state of the art that manufacture of a practical wire is difficult in the conventional approach of reducing the size of the diffusing pair of Nb and Al alloy.
The present invention was devised in consideration of the above circumstance, and it is an object of the present invention to provide a method of producing a Ge-added Nb3Al-based superconducting wire capable of realizing a practical Ge-added Nb3Al-based superconducting wire for use in strong magnetic fields having high critical current density and transport current in magnetic field regions of 21 T or higher.